The general goal is to understand the molecular basis of species differentiation by analyzing a recently duplicated gene, which is involved in hybrid male sterility. This speciation gene, Ods (Odysseus), was duplicated from the neuron-expressing gene, Unc4, during the evolution of Drosophila. Unc4 appears to retain the ancestral functions while Ods became specialized in a spermatogenic role. With this specialization, Ods has apparently diverged rapidly and such rapid divergence has made 0ds incompatible between species when hybrids are formed (Ting et al. 1998). The specific goal is to answer these questions: 1. Evolution of Ods sequence and expression: We will compare DNA sequences and expression patterns, both evolving rapidly, across Drosophila species. 2. Evolution of the normal functions of Ods: In order to understand Ods differentiation, we will use the new method of homology-dependent DNA insertion (Rong and Golic 2000) to replace the 0ds gene in D. melanogaster with those of D. mauritiana and D. simulans. Phenotypic and fitness changes due to the replacement will be assayed. 3. Evolution of the "abnormal" function of 0ds: 0ds-induced hybrid sterility is considered the "abnormal" function. By transgenic experiments, we will address the genetic mechanism by which 0ds sterilizes hybrid males.